Components for machine structures used for machines such as vehicles, construction machines, and the like (more specifically, steering components for a vehicle, drive shafts, chassis parts, and the like) are manufactured by cutting a steel bar so as to form the shape of a part. After forming the part shape, a component for a machine structure which requires strength and toughness is quenched and tempered (i.e. thermal refining) to ensure the strength and the toughness needed thereby. However, in order to decrease manufacturing costs for the parts and in order to protect the environment, there is demand for a process that omits heat treatment which consumes a huge amount of energy in recent years. Therefore, there is also demand for a process that omits the quenching and the tempering, i.e., the thermal refining process. It is considered that one way to omit the thermal refining process is to in-line quench a steel bar immediately after hot-rolling to be used as the material for a component of a machine structure and reheat the steel bar with sensible heat of the central part of the steel bar (i.e. self-tempering). However, if the quenching and the tempering are performed with the reheating, the hardening depth becomes uneven. If the hardening depth becomes uneven, warpage occurs in the steel bar. If a marked warpage occurs, it is necessary to correct the warpage and yield decreases due to shape failure, and thus, such marked warpage decreases production efficiency of the steel bar. In order to keep the production efficiency of the steel bar at a level preferable for industrial use, the amount of the warpage in the steel bar should be limited to less than 3 mm/m.
In the prior art of the steel bar, for example, a method in which a steel is directly quenched and tempered just after hot-rolling is disclosed in patent documents 1 to 7. However, patent document 1 relates to a rod mill round bar and does not consider induction hardenability. Patent document 2 proposes a method for enhancing the structure of a surface layer part of the steel by controlling the amount of cooling water. However, in the technique disclosed in the patent document 2, evenness of the hardening depth is not considered. The patent document 3 relates to a steel including 0.05 to 0.3% of C. The amount of C is insufficient for applying the induction hardening thereto as surface layer hardening treatment. Therefore, the steel disclosed in the patent document 3 does not have sufficient induction hardenability. Patent document 4 proposes a steel bar in which a surface layer part which is from the surface to a depth of 2 mm is controlled to be a sorbite structure and inner structure is controlled to be a ferrite and pearlite structure by direct quenching after hot working and self-tempering. However, in patent document 4, evenness of the hardening depth is not considered. The patent documents 5 to 7 disclose method for manufacturing, in which hot-rolling is performed during ferrite-austenite coexisting state (so called “dual phase rolling”). However, decarburizing easily occurs in steel obtained by the hot-rolling, and thus, induction hardenability of the steel disclosed in the patent documents 5 to 7 is insufficient.